Orphaned sea lion pups would take any milk and protection they can get, but decades of behavioral research has shown females viciously reject any unrelated pups who wander too closely.

A new genetic study of California sea lion populations, published Monday in the online journal PLoS ONE, now challenges the idea that sea lions are entirely cold to adoption.

Up to 17 percent of California sea lion females in populations off the coast of Mexico will take in an orphaned pup and raise it as their own, according to the new work. What’s more, the study’s authors witnessed females adopt orphaned pups and care for them season after season.

“Females are incredibly aggressive toward pups that aren’t theirs. They’ll bare teeth and bark, sometimes grab and toss pups that aren’t their own away,” said Ramona Flatz, a marine biologist at Arizona State University who made the discovery through an unrelated research effort. “That they adopt at all really surprised us. We didn’t think it happened.”

The islands of San Jorge (north) and Los Islotes (south). Map: PLoS ONE

California sea lions number more than 180,000 on the Pacific Coast, inhabiting a huge range from southern Alaskan shores down to the coast of Mexico. Females live for about 20 years and produce one pup each season, nursing it for 6 months.

Flatz and her fellow researchers spent three summers walking around sea lion rookeries on the Mexican islands of San Jorge and Los Islotes, located about 400 miles apart in the Gulf of California, aka Sea of Cortez. Their original mission: to sample the DNA of female-pup pairs for an entirely different study of the animal’s paternity.

To carry out the study, Flatz and her team used nets, electric razors, specialized crossbows and some guts. When the scientists found a nursing pup, they ran up and grabbed it by the back flippers, clipped a tissue sample from its toe, and gave it a special haircut to identify individual pups from far away.

Flatz captured pups when they were a couple days old to two months old and weighed from 18 to 26 pounds.

“However, they feel like about 50 lbs,” she wrote in an e-mail. The team spent five minutes or less processing each pup, without sedation. “They do bite, so speed was helpful.”

Before any pup was processed, however, the researchers used crossbows outfitted with nonlethal, tissue-collecting arrows for the roughly 250-pound mother sea lions.

“The arrows are attached by fishing line, so you shoot, it takes a little eraser chunk of skin out, and then you pull it back. Most females don’t even notice,” Flatz said.

Back in the lab, comparing the nursing pair’s DNA showed that more than 17 percent of females at Los Islotes island raised pups that weren’t their own, while 6 percent at San Jorge island adopted orphaned pups.

“I think those results are really thorough, but the thing that made me really confident these sea lions adopt were two individual cases, with photographs and behavioral observations,” Flatz said.

In one case she documented a female calling for its pup, which Flatz had given a haircut, for three days. “Then all of the sudden she has a pup” that was unmarked, Flatz said. The other case was a genetically distinct pup-female pair sampled over two seasons. “I have no doubt that these were adoption events,” she said.

Flatz can only speculate about California sea lion adoption, but thinks an evolution-backed maternal drive is responsible for the behavior in general. As for the high adoption rate as Los Islotes Island, she said it may be the work of people.

“I wouldn’t be surprised if tourism increases adoption. That could spook new mothers and cause them to take off for good, leaving their pups behind,” Flatz said. “Whatever is going on, it’s a great jumping-off point for more research.”